Devices which have been conventionally used for bone treatment include wires, plates, screws, pins, staples, clips, rods, etc., which are made of stainless steel, ceramic, etc. The bending strength of these conventional devices is adequately high (about 33 kgf/mm.sup.2 for devices made of SUS-316 stainless steel and about 25-50 kgf/mm.sup.2 for devices made of ceramics). However, they are not biodegradable and hence require a re-operation for their removal after healing. Furthermore, since they are more rigid than human bones, the use of these devices in vivo can cause problems such as bone shaving, local osteolysis due to continuous stimulation, a reduction in the strength of newly formed bones, and delay in the growth of regenerated bones.
Until now, several bone-treating devices made of biodegradable polymers have been proposed. However, these devices are inferior to the devices made of stainless steel, ceramic, etc. in terms of the bending strength and the stiffness to endure torsion, bending, etc. Furthermore, these biodegradable devices retain a therapeutically necessary strength only for less than 3 months (often about 1-2 months). It is therapeutically ideal that biodegradable bone-treating devices retain a therapeutically necessary strength for about 3 months and subsequently lose their strength rapidly by means of their decomposition in vivo, eventually leading to bioabsorption.
To improve the shortcomings of conventional biodegradable bone-treating devices, the applicants previously proposed an invention (Japanese Examined Patent Publication No. 1991-63901). That invention pertained to improving the initial strength and the strength-retaining characteristic by means of drawing and stretching molded biodegradable lactic acid polymers along their major axis in air or fluid under heating condition. The same proposal has also been made in the Japanese Unexamined Patent Publication No. 1991-29663.
However, the above-mentioned technique can not adequately improve the strength. It was difficult or impossible with that technique to manufacture bone-fusing devices whose initial strength is comparable to that of devices made of stainless steel or ceramic. As shown in the comparative examples 1-2, to be stated later, the density and the bending strength of an cylindrical product, manufactured by melting and extruding poly-L-lactic acid (about 400,000 in viscosity-average molecular weight), was 1,250 g/cm.sup.3 and 22.0 kgf/mm.sup.2, respectively, when the drawing was performed at a ratio of 4:1 in an oil bath at 140.degree. C. They were 1.250 g/cm.sup.3 and 22.6 kgf/mm.sup.2 when the draw ratio was 9.8:1. These values indicate a limit of that technique. The unsatisfactory results may be attributable to the enlargement of the slight voids (which had been produced during molding) due to the drawing procedure at elevated temperatures after molding at normal pressure. This explanation is supported by the finding that increase of drawing ratio causes no increase in the density based on the molecular arrangement. None of the previously proposed bone-treating devices, made of biodegradable polymers, had a density and bending strength higher than the above-mentioned levels.
The present invention is purposed to resolve the above-mentioned problems, i.e., to provide those bone-treating devices made of biodegradable polymers which have an initial strength comparable to that of similar devices made of stainless steel or ceramic; which retain a therapeutically necessary strength for adequate periods; and which rapidly lose their strength by means of hydrolysis and are eventually absorbed in vivo after elapse of the therapeutically necessary periods.